Solid stabilizer, steel material for solid stabilizer, and manufacturing method of solid stabilizer

ABSTRACT

A steel material for a solid stabilizer which has high bendability, high hardenability, and high quenching crack resistance, a solid stabilizer having high strength, and a manufacturing method of the solid stabilizer. The steel material for the solid stabilizer contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B. The steel material for the solid stabilizer satisfies a condition of formula (1) below. Hardness in a radial center portion of the steel material for the solid stabilizer after tempering is 400 HV or more, and a martensite ratio in the radial center portion after the tempering is 80% or more. 
       1.24&lt;(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B2)×240}&lt;1.7   (1)
 
     It is an object to provide a steel material for a solid stabilizer which has high bendability, high hardenability, and high quenching crack resistance, a solid stabilizer having high strength, and a manufacturing method of the solid stabilizer. 
     The steel material for the solid stabilizer contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B. The steel material for the solid stabilizer satisfies a condition of formula (1) below. Hardness in a radial center portion of the steel material for the solid stabilizer after tempering is 400 HV or more, and a martensite ratio in the radial center portion after the tempering is 80% or more. 
       1.24&lt;(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B 2 )×240}&lt;1.7   (1)

TECHNICAL FIELD

The present invention relates to solid stabilizers that ensure stability during running of a vehicle, steel materials for the solid stabilizers, and manufacturing methods of the solid stabilizers.

BACKGROUND ART

Stabilizers couples suspension arms that are located on both sides of a vehicle in the vehicle width direction. The stabilizers suppress roll of the vehicle during running of the vehicle. The stabilizers include solid stabilizers that are fabricated from a solid steel material, and hollow stabilizers that are fabricated from a hollow steel material. In recent years, the hollow stabilizers have been increasingly used in order to reduce the vehicle weight. For example, Patent Document 1 discloses an electric resistance welded steel pipe for a hollow stabilizer, and Patent Document 2 discloses a manufacturing method of a hollow stabilizer,

RELATED ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Patent Application Publication No. 2004-11009 (JP 2004-11009 A)

[Patent Document 2] Japanese Patent Application Publication No. 2005-76047 (JP 2005-76047 A)

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, desired strength of the solid stabilizers can be more easily ensured compared to the hollow stabilizers. Moreover, the solid stabilizers can be manufactured at low cost. Accordingly, the solid stabilizers are still often used despite the growing demand for the hollow stabilizers. These two types of stabilizers are different in that one of the two types of stabilizers has a hollow structure and the other has a solid structure. Accordingly, properties required for the steel material are also different between the two types of stabilizers. The differences in the required properties will be specifically described below.

A manufacturing method of a stabilizer has a forming step, a quenching step, and a tempering step. In the forming step, a steel material is bent into the shape of a stabilizer. Steel materials for hollow stabilizers have a hollow in their radial center portions. Accordingly, the steel materials for hollow stabilizers essentially have low resistance to bending deformation. Thus, the steel materials for hollow stabilizers have high bendability. On the other hand, steel materials for solid stabilizers are solid all the way to their radial center portions. Accordingly, the steel materials for solid stabilizers essentially have high resistance to bending deformation. Thus, the steel materials for solid stabilizers have poor bendability and have a large amount of springback. Thus, requirements for springback are stricter when manufacturing the solid stabilizers, compared to the case of manufacturing the hollow stabilizers. Moreover, higher strength is required for the solid stabilizers than for the hollow stabilizers, which increases the springback in the solid stabilizers.

Accordingly, in manufacturing of the hollow stabilizers, the springback is not large enough to cause a problem. However, in manufacturing of the solid stabilizers, a material etc. need to be selected in view of the amount of springback as well.

In the quenching step, the steel material that has been bent is hardened. The hollow stabilizers have a hollow in their radial center portions. Thus, the steel materials for hollow stabilizers are not required to have high hardenability. On the other hand, the radial center portions of the solid stabilizers are filled with the steel material. Thus, the steel materials for solid stabilizers are required to have hardenability that is high enough for the solid materials to be hardened to their radial center portions.

The hollow stabilizers have a hollow in their radial center portions. Accordingly, when rapidly cooling the hollow stabilizers in the quenching step, the difference in amount of shrinkage (the difference in cooling speed) is small between the outer peripheral surface and the inner peripheral surface. Thus, quenching cracks are less likely to occur in the hollow stabilizers. On the other hand, the radial center portions of the solid stabilizers are filled with the steel material. Accordingly, when rapidly cooling the solid stabilizers in the quenching step, the difference in amount of shrinkage (the difference in cooling speed) is large between the outer peripheral surface and the radial center portion. Thus, quenching cracks tend to occur in the solid stabilizers. Therefore, the steel materials for solid stabilizers are required to be more resistant to quenching cracks than the steel materials for hollow stabilizers. That is, higher quenching crack resistance is required for the steel materials for solid stabilizers than for the steel materials for hollow stabilizers.

Moreover, as described above, desired strength of the solid stabilizers can be more easily ensured compared to the hollow stabilizers. Thus, the solid stabilizers are often used for vehicles for which high strength is required. However, in order to improve the strength, the content of C etc. that is effective in improving the strength must be increased, which causes degradation in the bendability and the quenching crack resistance. Accordingly, intended properties cannot be obtained if the technique used for the hollow stabilizers is used as it is to manufacture the solid stabilizers having higher strength than that of the hollow stabilizers.

As described above, the properties required for the steel materials for hollow stabilizers are completely different from those required for the steel materials for solid stabilizers. Accordingly, not all the requirements for the properties such as bendability, hardenability, and quenching crack resistance can be satisfied even if the steel materials for hollow stabilizers are used as they are as the steel materials for solid stabilizers.

A solid stabilizer, a steel material for the solid stabilizer, and a manufacturing method of the solid stabilizer according to the present invention were completed in view of the above problems. It is an object of the present invention to provide a solid stabilizer that has high strength and does not cause the above problems in terms of the bendability, the hardenability, and the quenching crack resistance when being manufactured, a steel material for the solid stabilizer, and a manufacturing method of the solid stabilizer.

Means for Solving the Problem

(1) In order to achieve the above object, a solid stabilizer according to the present invention is a solid stabilizer fabricated by cold forming, quenching, and tempering a steel material for the solid stabilizer. The solid stabilizer is characterized in that the steel material for the solid stabilizer contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B, and satisfies a condition of formula (1) below, remainder of the steel material for the solid stabilizer is formed of Fe and an unavoidable impurity, and hardness in a radial center portion of the solid stabilizer after the tempering is 400 HV or more, and a martensite ratio in the radial center portion of the solid stabilizer after the tempering is 80% or more.

1.24<(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B²)×240}<1.7   (1)

(2) Preferably, in the above configuration of (1), a lower limit of the C is 0.25%, an upper limit of the Mn is 1.00%, and a lower limit of the formula (1) is 1.4.

(3) Preferably, in the above configuration of (1) or (2), a condition of formula (2) below is satisfied.

(Si/C)<1.5   (2)

(4) Preferably, in the above configuration of any one of (1) to (3), after the tempering, tensile strength is 1,200 MPa or more, 0.2% proof stress is 1,100 MPa or more, and an impact value at room temperature is 70 J/cm² or more. According to this configuration, a solid stabilizer having both high strength and high toughness can be obtained.

(5) In order to achieve the above object, a steel material for a solid stabilizer is characterized in that the steel material for the solid stabilizer contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B, and satisfies a condition of formula (1) below, remainder of the steel material for the solid stabilizer is formed of Fe and an unavoidable impurity, and in finish rolling, the steel material for the solid stabilizer is rolled at a heating temperature of 1,000° C. or less, and hardness of the steel material for the solid stabilizer after the rolling is 200 HV or less.

1.24<(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B²)×240}<1.7   (1).

(6) Preferably, in the above configuration of (5), a lower limit of the C is 0.25%, an upper limit of the Mn is 1.00%, and a lower limit of the formula (1) is 1.4.

(7) Preferably, in the above configuration of (5) or (6), a condition of formula (2) below is satisfied.

(Si/C)<1.5   (2)

(8) In order to achieve the above object, a method for manufacturing a solid stabilizer according to the present invention is characterized by including: a forming step of cold bending a steel material for the solid stabilizer that contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B, and that satisfies a condition of formula (1) below, wherein remainder of the steel material for the solid stabilizer is formed of Fe and an unavoidable impurity, and in finish rolling, the steel material for the solid stabilizer is rolled at a heating temperature of 1,000° C. or less, and hardness of the steel material for the solid stabilizer after the rolling is 200 HV or less, the method further including: a quenching step of quenching the steel material for the solid stabilizer after the forming; and a tempering step of tempering the steel material for the solid stabilizer after the quenching.

1.24<(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B²)×240}<1.7   (1)

(9) Preferably, in the above configuration of (8), a lower limit of the C is 0.25%, an upper limit of the Mn is 1.00%, and a lower limit of the formula (1) is 1.4.

(10) Preferably, in the above configuration of (8) or (9), a condition of formula (2) below is satisfied.

(Si/C)<1.5   (2)

Effects of the Invention

In the steel material for the solid stabilizer according to the present invention, components thereof are optimized, whereby performance that satisfies mechanical properties such as strength, toughness, etc. can be obtained while ensuring required hardenability, quenching crack resistance, and bendability. The performance level of the steel material for the solid stabilizer according to the present invention is not necessarily obviously higher than that of conventional steels, if the bendability, the quenching crack resistance, and the mechanical properties of the steel material according to the present invention are individually compared with those of the conventional steels. However, the conventional steels satisfy the required levels of some of the properties, although do not necessarily satisfy other properties. In contrast, the most outstanding feature of the present invention is that optimal product design was developed so as to satisfy all the required properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a solid stabilizer according to an embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

An embodiment of a solid stabilizer, a steel material for the solid stabilizer, and a manufacturing method of the solid stabilizer according to the present invention will be described below.

<Steel Material for Solid Stabilizer>

[Content of Each Component in Steel Material for Solid Stabilizer]

First, the content of each component in the steel material for the solid stabilizer will be described.

(C)

C is a component that is essential to ensure strength required as a solid stabilizer after quenching and tempering. The C content is 0.24 mass % (hereinafter abbreviated as “%” as required) or more for the following reasons. If the C content is less than 0.24%, the strength of the solid stabilizer is reduced, and also hardenability of a steel material for the solid stabilizer is reduced. For similar reasons, it is preferable that the C content be 0.25% or more. The C content is 0.40% or less for the following reasons. If the C content exceeds 0.40%, quenching crack resistance of the steel material for the solid stabilizer is reduced, and also toughness after tempering is reduced. Moreover, hardness of the steel material for the solid stabilizer after finish rolling (before cold forming) is increased, and bendability during cold forming is reduced.

(Si)

Si functions as a deoxidizes at the time of steel making. The Si content is 0.15% or more for the following reasons. If the Si content is less than 0.15%, the hardenability of the steel material for the solid stabilizer is reduced, and also the strength of the solid stabilizer is reduced. The Si content is 0.40% or less for the following reasons. If the Si content exceeds 0.40%, the quenching crack resistance of the steel material for the solid stabilizer is reduced. Moreover, the hardness of the steel material for the solid stabilizer after finish rolling (before cold forming) is increased, and the bendability during cold forming is reduced.

(Mn)

Mn is added to improve the hardenability of the steel material for the solid stabilizer. The Mn content is 0.50% or more for the following reasons. If the Mn content is less than 0.50%, the hardenability of the steel material for the solid stabilizer is reduced, and also the strength of the solid stabilizer is reduced. The Mn content is 1.20% or less for the following reasons. If the Mn content exceeds 1.20%, the quenching crack resistance of the steel material for the solid stabilizer is reduced. Moreover, the hardness of the steel material for the solid stabilizer after finish rolling (before cold forming) is increased, and the bendability during cold forming is reduced. For similar reasons, it is preferable that the Mn content be 1.00% or less.

(P)

It is preferable that the P content be as low as possible. The P content is 0.03% or less for the following reason. If the P content exceeds 0.03%, the toughness after tempering is reduced.

(Cr)

Like Mn, Cr is added to improve the hardenability of the steel material for the solid stabilizer. The Cr content is 0.30% or less for the following reason. If the Cr content exceeds 0.30%, the quenching crack resistance of the steel material for the solid stabilizer is reduced. Moreover, the hardness of the steel material for the solid stabilizer after finish rolling (before cold forming) is increased, and the bendability during cold forming is reduced.

(B)

Like Mn and Cr, B has an effect of improving the hardenability of the steel material for the solid stabilizer. Moreover, B has an effect of improving grain boundary strength. The B content is 0.0010% or more for the following reasons. If the B content is less than 0.0010%, the hardenability of the steel material for the solid stabilizer is reduced, and also the strength of the solid stabilizer is reduced. The B content is 0.0030% or less for the following reason. The effects produced by adding B (the effect of improving the hardenability and the effect of improving the strength) are gradually saturated as the amount of B that is added is increased. Thus, the effects are saturated even if more than 0.0030% of B is added. In view of this, in Formula (1) described below as well, a quadratic term is set in addition to a linear term for the B content.

(Ti)

B tends to bond with N contained in steel. If B bonds with N to produce BN, the effects produced by adding B are not obtained. Accordingly, Ti is added to allow Ti and N to produce TiN, whereby the effects produced by adding B are ensured.

The Ti content is 0.01% or more for the following reason. If the Ti content is less than 0.01%, it is difficult to ensure the effects produced by adding B. The Ti content is 0.03% or less for the following reason. If the Ti content exceeds 0.03%, coarse TiN tends to be produced, and toughness is reduced.

In addition to the above components, the steel material for the solid stabilizer according to the present invention may contain, as an impurity, an amount of Al (about 0.040% or less) which is required for a deoxidizing process essential for steel manufacturing.

[Formula (1)]

Formula (1) will be described below. Formula (1) is an empiric formula obtained by multivariate analysis of experimental data. The hardenability and quenching crack resistance of the steel material for the solid stabilizer can be optimized by setting the content of each component according to Formula (1).

The numerical value obtained by substituting the content of each component (the value in %; e.g., 0.25 in the case of 0.25%) into Formula (1) is higher than 1.24 for the following reasons. If this numerical value is 1.24 or less, the hardenability is not high enough for use as a solid stabilizer, and it is difficult to ensure a martensite ratio of 80% or more all the way to the radial center portion after quenching, whereby the strength of the solid stabilizer is reduced. For similar reasons, it is preferable that the numerical value of Formula (1) be higher than 1.4. The numerical value obtained by substituting the content of each component into Formula (1) is less than 1.7 for the following reason. If this numerical value is 1.7 or more, there is a possibility that quenching cracks cannot be completely prevented in the case of manufacturing solid stabilizers that are more susceptible to quenching cracks than hollow stabilizers for the reason described above. Thus, by using Formula (1), acceptable levels of hardenability and quenching crack resistance can be ensured when manufacturing the solid stabilizers.

[Formula (2)]

Formula (2) will be described below. Formula (2) is an empiric formula obtained by multivariate analysis of experimental data. Surface hardness of the steel material for the solid stabilizer can be optimized by setting the contents of Si and C according to Formula (2).

The numerical value of Formula (2) is less than 1.5 for the following reasons. If this numerical value is 1.5 or more, the amount of Si is large relative to C, whereby decarburization tends to occur. That is, the strength of the surface of the solid stabilizer becomes lower compared, to the hardness of the inner part of the solid stabilizer. Thus, reduction in strength of the surface of the solid stabilizer can be suppressed by using Formula (2).

[Heating Temperature, Hardness after Rolling]

The heating temperature during finish rolling is 1,000° C. or less for the following reason. If the heating temperature is higher than 1,000° C., hardness after rolling is increased, and cold bendability of the steel material for the solid stabilizer is reduced. Specifically, springback is increased, which increases a variation in shape after bending. The hardness after rolling is 200 HV or less for the following reason. If the hardness after rolling is 200 HV or less, the amount of springback during bending can be reduced to a target value or less.

<Solid Stabilizer>

The content of each component, Formula (1), and Formula (2) of the steel material for the stabilizer as a material for the solid stabilizer are as described above.

FIG. 1 is a perspective view of the solid stabilizer according to the embodiment of the present invention. As shown in FIG. 1, the overall shape of a solid stabilizer 1 is a U-shape. The solid stabilizer 1 includes a torsion portion 10 and a pair of arm portions 11. The torsion portion 10 extends in a vehicle width direction. The pair of arm portions 11 are coupled to both axial ends of the torsion portion 10. A pair of rings 12 are fixed by crimping near both ends in the vehicle width direction of the torsion portion 11. A pair of bushes 13 are fitted to the outer sides in the vehicle width direction of the pair of rings 12. The bushes 13 are fixed to a vehicle body (not shown). Eye portions 110 are provided at respective distal ends of the pair of arm portions 11. The pair of eye portions 110 are each coupled to a suspension arm (not shown).

[Hardness after Tempering, Martensite Ratio]

The hardness after tempering and the martensite ratio are determined in the radial center portion for the following reasons. Solid stabilizers need to have a structure with their radial center portions hardened. If hardenability is not high enough, the solid stabilizers do not have a structure with their radial center portions hardened, whereby the hardness of the solid stabilizers is reduced. Accordingly, if the solid stabilizers have both an acceptable structure and acceptable hardness in their radial center portions, a martensite ratio of 80% or more can be ensured in the portions other than the radial center portion, such as the surface.

The hardness in the radial center portion after tempering is 400 HV or more for the following reason. Since the object of the present invention is to ensure the strength that is the same as or higher than that of the conventional hollow stabilizers, the hardness of less than 400 HV is not high enough to ensure the strength. Similarly, the martensite ratio in the radial center portion after tempering is 80% or more for the following reason. If this martensite ratio is less than 80%, the intended strength cannot be obtained.

[Tensile Strength, 0.2% Proof Stress, Impact Value]

Tensile strength is 1,200 MPa or more, 0.2% proof stress is 1,100 MPa or more, and an impact value at room temperature is 70 J/cm² or more for the following reason. If the tensile strength, the 0.2% proof stress, and the impact value are less than these lower limits, high strength and high toughness, which are required for solid stabilizers, cannot be achieved at the same time.

That is, the solid stabilizers are often used for vehicles for which high strength and high toughness are required, compared to hollow stabilizers. Accordingly, if the tensile strength, the 0.2% proof stress, and the impact value are less than the above lower limits, there is a possibility that strict requirements for the solid stabilizers cannot be satisfied.

<Manufacturing Method of Solid Stabilizer>

The manufacturing method of the solid stabilizer includes a forming step, a quenching step, and a tempering step. In the forming step, the steel material for the solid stabilizer after finish rolling is subjected to cold bending so that the steel material for the solid stabilizer has a shape of the solid stabilizer to be fabricated. In the quenching step, the steel material for the solid stabilizer is first heated to austenitize the structure of the steel material, and is then rapidly cooled to obtain a hard martensite structure. In the following tempering step, toughness of the steel material for the solid stabilizer is improved.

The content of each component in the steel material for the solid stabilizer, Formula (1), Formula (2), the heating temperature during finish rolling, and the hardness after finish rolling are as described above.

<Others>

The embodiment of the solid stabilizer, the steel material for the solid stabilizer, and the manufacturing method of the solid stabilizer according to the present invention are described above. However, embodiments are not particularly limited to the above embodiment. Various modifications and improvements can be made by those skilled in the art.

For example, although a heating method in the quenching step of the manufacturing method of the solid stabilizer is not particularly limited, furnace heating, electrical heating, etc. may be used as the heating method. Although a cooling medium for the rapid cooling in the quenching step is not particularly limited, water, a polymer solution, etc. may be used as the cooling medium. The temperature patterns of heating and cooling in the quenching step and the tempering step are not particularly limited.

Moreover, the bending in the forming step is not particularly limited. For example, cold bending can be performed by using an NC bender, a bending die, etc.

EXAMPLE 1

Evaluation tests for quenching crack resistance, cold bendability, strength, durability, and toughness, which were carried out on stabilizers and steel materials for the stabilizers, will be described below.

<Manufacturing Method of Samples>

First, a manufacturing method of samples (stabilizers of Examples 1 to 8 and Comparative Examples 1 to 7, 9 to 13) will be described. The manufacturing method of the samples includes a hot forging step, a forming step, a quenching step, a tempering step, and a finishing step.

In the hot forging step, a steel material was first cut to a predetermined length. Then, both axial ends of the cut steel material were heated and hot forged, and holes were formed therein. As shown in FIG. 1, the pair of eye portions 110 were formed in both axial ends of the steel material in this manner.

In the forming step, cold bending was performed on the steel material. Specifically, the steel material was bent into a U-shape. As shown in FIG. 1, the torsion portion 10 and the pair of arm portions 11 were formed in this manner.

In the quenching step, the pair of eye portions 110 in the steel material were first clamped. Then, a current was applied between the pair of eye portions 110 to heat the steel material to a quenching temperature of 970° C. Thereafter, the steel material was rapidly cooled with water.

In the tempering step, the steel material was heated again and slowly cooled. The highest heating temperature (tempering temperature) was adjusted such that a target hardness of 420 HV is achieved in the radial center portion of the steel material after tempering. However, in Comparative Example 10 described below, the hardness increased to only 320 HV in the quenching step. Accordingly, it was determined that it would be impossible to adjust the hardness to 420 HV in the tempering step. Since there was a possibility that performing tempering might further reduce the hardness and thus might further increase the difference between the actual hardness and the target value, tempering was not performed in Comparative Example 10. In Example 7, a painting step that involves heating of the steel material was performed as the tempering step as well. The painting temperature, namely the tempering temperature was 200° C.

In the finishing step, the shape of the steel material was first finely adjusted, and then the surface of the steel material was subjected to shot peening. Thereafter, the surface of the steel material was painted. Lastly, as shown in FIG. 1., the pair of rings 12 were fixed by crimping to the torsion portion 10. The samples were manufactured in this manner.

Comparative Example 8 is a sample that is used to evaluate the difference in possibility of quenching cracks between hollow and solid stabilizers. Only the quenching crack resistance was evaluated for Comparative Example 8. An evaluation method will be described later.

<Compositions of Samples>

The compositions of the samples will be described below. Table 1 shows data about the components, manufacturing conditions, evaluation items (quenching crack resistance, cold bendability, strength, durability, and toughness) of Examples 1 to 8 and Comparative Examples 1 to 13.

In the table, the numerical values denoted with the symbol “*” indicate that they are out of the numerical value ranges of the components and properties defined in the claims. The numerical values denoted with the symbol “#” means that the corresponding properties, although not defined by the claims, have a very undesirable value.

TABLE 1 Manufacturing Conditions Quenching Rolling Crack Chemical Composition Heating Quenching Tempering Resistance Formula Formula Temperature Temperature Temperature Quenching No. C(%) Si(%) Mn(%) Cr(%) Ti(%) B(%) (1) (2) (° C.) (° C.) (° C.) Crack Example 1 0.33 0.23 0.82 0.16 0.03 0.0024 1.56 0.70 900 970 380 ◯ 2 0.26 0.25 0.96 0.15 0.02 0.0019 1.41 0.96 900 970 300 ◯ 3 0.36 0.27 0.92 0.14 0.02 0.0022 1.69 0.75 900 970 400 ◯ 4 0.32 0.35 0.72 0.23 0.01 0.0014 1.44 1.09 900 970 320 ◯ 5 0.33 0.23 0.82 0.16 0.03 0.0024 1.56 0.70 1000 970 380 ◯ 6 0.24 0.21 0.79 0.25 0.02 0.0020 1.31 0.88 900 970 240 ◯ 7 0.24 0.15 0.89 0.22 0.01 0.0012 1.25 0.63 900 970 (200) ◯ 8 0.28 0.31 1.18 0.19 0.02 0.0028 1.64 1.11 900 970 280 ◯ Comparative 1 *0.43 0.20 0.60 0.13 0.02 0.0021 1.69 0.47 900 970 420 ◯ Example 2 0.33 0.25 0.84 *0.40 0.03 0.0025 *1.71 0.76 900 970 400 X 3 0.34 *0.50 0.82 0.16 0.03 0.0021 1.61 1.47 900 970 360 ◯ 4 0.34 0.24 1.06 0.17 0.03 0.0024 *1.73 0.71 900 970 360 X 5 0.35 0.24 0.81 0.16 *0.05 0.0024 1.61 0.69 900 970 400 ◯ 6 0.24 0.21 0.56 0.29 0.02 0.0025 *1.22 0.88 900 970 240 ◯ 7 0.35 0.25 *1.40 0.02 0.02 0.0030 *1.84 0.71 900 970 420 X 8 0.33 0.25 *1.40 0.02 0.02 0.0030 *1.84 0.71 — 970 — ◯ 9 *0.23 0.40 0.95 0.25 0.02 0.0021 1.41 *1.74 900 970 200 ◯ 10 0.33 0.25 0.80 0.15 *— *— *1.07 0.76 900 970  0 ◯ 11 *0.54 0.28 0.83 *0.85 *— *— *1.78 0.52 900 970 490 X 12 0.33 0.23 0.82 0.16 0.03 0.0024 1.56 0.70 *1100 970 380 ◯ 13 0.36 0.27 0.92 0.14 0.02 0.0022 1.69 0.75 *1100 970 400 ◯ Cold Bendability Hardness Strength Durability Toughness after Surface Martensite 0.2% Proof Tensile Durability Impact Rolling Hardness Hardness Ratio Stress Strength Test Value No. (HV) Springback (HV) (HV) (%) (MPa) (MPa) (times) (J/cm²) Example 1 178 0.71 418 423 100 1221 1390 87700 122 2 169 0.64 406 416 85 1148 1355 76300 132 3 194 0.84 412 420 100 1236 1369 88200 98 4 176 0.74 405 420 90 1189 1378 81300 118 5 198 0.94 420 425 100 1232 1404 88200 114 6 167 0.52 421 425 80 1121 1385 75400 112 7 165 0.60 425 430 80 1130 1418 78900 108 8 195 0.65 415 430 100 1254 1420 91200 95 Comparative 1 *213 #1.07 422 427 100 1272 1402 91300 *67 Example 2 188 0.77 414 423 100 1229 1377 79800 102 3 *205 #1.06 396 422 100 1243 1382 83400 124 4 196 0.93 412 419 100 1232 1373 75500 112 5 184 #1.16 408 412 100 1208 1346 78300 *65 6 165 0.82 405 413 *75 *1078 1352 *40300 123 7 *218 #1.09 410 420 100 1243 1377 91000 85 8 — — — — — — — — — 9 168 0.67 375 410 80 *1053 1350 #51200 126 10 176 0.76 318 *320  *60 *352 *1078  #9800 *62 11 *286 #1.38 419 423 100 1232 1380 91600 *23 12 *210 #1.03 412 421 100 1211 1382 89200 119 13 *234 #1.17 409 418 100 1221 1350 89200 95

Characteristics of Examples 1 to 8 and Comparative Examples 1 to 13 will be briefly described below. Examples 1 to 8 are solid stabilizers of the present invention. In Comparative Example 1, the C content is higher than the upper limit of the composition range of the present invention. In Comparative Example 2, the Cr content is higher than the upper limit of the composition range of the present invention, and the numerical value of Formula (1) is greater than the upper limit of the composition range of the present invention. In Comparative Example 3, the Si content is higher than the upper limit of the composition range of the present invention. In Comparative Example 4, the numerical value of Formula (1) is greater than the upper limit of the composition range of the present invention. In Comparative Example 5, the Ti content is higher than the upper limit of the composition range of the present invention. In Comparative Example 6, the numerical value of Formula (1) is less than the lower limit of the composition range of the present invention. Comparative Example 6 is a solid stabilizer manufactured by using the same material as the steel material that is conventionally generally used for solid stabilizers. In Comparative Example 7, the Mn content is higher than the upper limit of the composition range of the present invention, and the numerical value of Formula (1) is greater than the upper limit of the composition range of the present invention. Comparative Example 7 is a solid stabilizer formed of the same steel material as that of a base steel pipe C in Table 1 of Patent Document 2. That is, substantially the same steel material as that disclosed in Patent Document 2 as a material for a solid stabilizer was prepared, and the solid stabilizer was manufactured from this steel material. Comparative Example 8 will be described later. In Comparative Example 9, the C content is lower than the lower limit of the composition range of the present invention, and the numerical value of Formula (2) is greater than the upper limit of the composition range of the present invention. In Comparative Example 10, the Ti and B contents are lower than their respective lower limits for the composition range of the present invention, and the numerical value of Formula (1) is less than the lower limit of the composition range of the present invention. Comparative Example 10 is a solid stabilizer formed of a steel material corresponding to a KS carbon steel S33C. In Comparative Example 11, the C and Cr contents are higher than their respective upper limits for the composition range of the present invention, the Ti and B contents are lower than their respective lower limits for the composition range of the present invention, and the numerical value of Formula (1) is greater than the upper limit of the composition range of the present invention. Comparative Example 11 is a solid stabilizer fowled of a steel material corresponding to a HS spring steel SUP9. Comparative Example 12 is a solid stabilizer formed of the same steel material as that of Example 1. The heating temperature during finish rolling of the steel material is higher than the upper limit of the present invention. Comparative Example 13 is a solid stabilizer formed of the same steel material as that of Example 3. The heating temperature during finish rolling of the steel material is higher than the upper limit of the present invention.

Although not shown in Table 1, all the steel materials used for the examples are deoxidized steel materials, and thus contain a small amount (0.010 to 0.035%) of Al. Although not shown in Table 1, all the steel materials used for the examples contain 0.03% or less of P.

<Evaluation Items and Evaluation Method>

The evaluation items and the evaluation method will be described below with reference to Table 1. Only the quenching crack resistance was evaluated for Comparative Example 8.

[Quenching Crack Resistance]

The quenching crack resistance was evaluated by using thirty test pieces (having a diameter of 26 mm and a length of 100 mm, and having a V-shaped notch with a depth of 1 mm) that had been cut out from a steel material after finish rolling. That is, thirty test pieces were used per sample. Thirty test pieces, each having a diameter of 26 mm and a length of 100 mm, having a V-shaped notch with a depth of 1 mm, and having a thickness of 4 mm in the radial direction, were used for Comparative Example 8.

The thirty test pieces were first held at a quenching temperature of 970° C. for thirty minutes, and were then cooled with water. After cooling with water, the test pieces were observed. The symbol “X” represents the case where quenching cracks were observed in at least one of the thirty test pieces, and the symbol “0” represents the case where quenching cracks were not observed in any of the thirty test pieces.

The V-shaped notch was formed in the test pieces for the following reason. Essentially, quenching cracks are not acceptable even if they occur in only some of the mass-produced products, and occur rather infrequently. Thus, in the case of using a small number of test pieces, there may be no difference among them. Moreover, in many cases, actual quenching cracks occur due to fractures such as small cuts. Therefore, it is required that no quenching crack occur in the products even if they have such a fracture.

[Cold Bendability]

(Hardness after Rolling)

The hardness of the steel material after rolling was evaluated by Vickers hardness (JIS Z 2244 HV10) of the test pieces that had been cut out from the steel materials after finish rolling.

(Springback)

The cold bendability was evaluated by springback. This is because a variation in shape after bending increases as the springback increases, as described above.

The springback was evaluated by using the ratio (actual angle/designed angle) of an actual angle between the torsion portion 10 and the arm portion 11 after actual bending to a rotation angle of a bender head of an NC bender (a designed angle between the torsion portion 10 and the arm portion 11) when performing a bending process on the steel material.

Specifically, the amount of springback in the case where a solid stabilizer was manufactured by using Comparative Example 6 that is a typical example of a conventionally used steel material for hollow stabilizers is defined as R01, and the amount of springback of Comparative Example 11 that corresponds to the most average components of the conventionally used JIS spring steel material SUPS is defined as R02. The average of R01 and R02 is defined as R0. The amount of springback of each sample is defined as R1. The ratio of R1 to R0, namely R1/R0, is shown in Table 1.

As described above, a variation after bending increases as the springback increases. Thus, a target value of this variation was set to 1.0 or less, and evaluation of the springback was conducted based on this target value. This is because it is known from the past actual manufacturing data that the variation can be suppressed to an acceptable level or less in the bending process during manufacturing of the solid stabilizer, if the amount of springback can be reduced to at most about the average of the amount of springback of the steel material for hollow stabilizers such as Comparative Example 6 and the amount of springback of the JIS spring steel material SUP9 such as Comparative Example 11.

[Strength]

(Surface Hardness)

The surface hardness of the stabilizer was evaluated by the Vickers hardness (JIS Z 2244 HVIO) of the test pieces that had been cut out from the stabilizers.

(Hardness)

The hardness of the stabilizer was evaluated by the Vickers hardness (JIS Z 2244 HV10) of the test pieces that had been cut out from the stabilizers.

(Martensite Ratio)

The martensite ratio was evaluated by observing with an optical microscope (400×) the structures of the radial center portions of the test pieces that had been cut out from the stabilizers.

(0.2% Proof Stress)

As in the tensile strength described below, a tensile test (JIS Z 2241) was conducted on the test pieces (No. 14A test pieces, JIS Z 2201) that had been cut out from the stabilizers, and the 0.2% proof stress was evaluated by the stress that caused 0.2% of permanent strain upon removal of the load.

(Tensile Strength)

The tensile strength was evaluated by conducting a tensile test (JIS Z 2241) on the test pieces (No. 14A test pieces, JIS Z 2201) that had been cut out from the stabilizers.

[Durability]

The durability was evaluated by a durability test conducted on the stabilizers. As shown in FIG. 1, the stabilizer 1 has a diameter of 26 mm. The distance between the pair of bushes 13 is 490 mm. The distance between the pair of eye portions 110 is 820 mm.

In the durability test, the pair of bushes 13 were first fixed to a jig (not shown). Then, the pair of eye portions 110 were vibrated alternately in the opposite vertical directions. The vibration frequency was 2 Hz. The vibration stroke (between the bottom dead center and the top dead center) was 70 mm. The durability was evaluated by the number of cycles until the stabilizer was fractured.

[Toughness]

The toughness was evaluated by conducting a Charpy impact test (JIS Z 2242) at 20° C. on the test pieces (JIS No. 3, 2-nun U-notch test pieces) that had been cut out from the stabilizers.

<Test Results>

The test results will be described below with reference to Table 1. According to Examples 1 to 8, satisfactory evaluation results were obtained for all the evaluation items in both the steel materials and the stabilizers.

In contrast, according to Comparative Examples 1 to 13, satisfactory evaluation results were obtained in some of the evaluation items, but not all of the evaluation items. This will be described briefly. Cold bendability was low in Comparative Example 1. Toughness was also low in Comparative Example 1. Quenching cracks occurred in Comparative Example 2. Cold bendability was low in Comparative Example 3. Quenching cracks occurred in Comparative Example 4. Springback was large in Comparative Example 5. Moreover, toughness was low in Comparative Example 5. The martensite ratio was low in Comparative Example 6. 0.2% proof stress and durability were also low in Comparative Example 6. Quenching cracks occurred in Comparative Example 7. Moreover, hardness after rolling was high and cold bendability was low in Comparative Example 7. 0.2% proof stress was low in Comparative Example 9. Durability was also low in Comparative Example 9. Strength (hardness, martensite ratio, 0.2% proof stress, and tensile strength) was low in Comparative Example 10. Durability and toughness were also low in Comparative Example 10. Hardness after rolling was high and cold bendability was low in Comparative Example 11. Moreover, toughness was low in Comparative Example 11. Hardness after rolling was high and cold bendability was low in Comparative Example 12. Hardness after rolling was high and cold bendability was low in Comparative Example 13.

Of Comparative Examples 1 to 13, Comparative Example 6 is a solid stabilizer manufactured by using the steel material that is used for hollow stabilizers. This shows that satisfactory evaluation results are not obtained for any evaluation items even if the steel material for hollow stabilizers is used as it is for solid stabilizers.

A hollow test piece using the same steel material as that of Comparative Example 7 was prepared to evaluate the quenching crack resistance of Comparative Example 8. According to the evaluation result, quenching cracks occurred in the solid Comparative Example 7 whereas no quenching cracks occurred in the hollow

Comparative Example 8. This shows that in the case of using the same steel material, quenching cracks may occur in solid stabilizers even if no quenching cracks occur in hollow stabilizers. In the present invention, the components are designed also in view of the fact that quenching cracks are more likely to occur in the solid stabilizers than in the hollow stabilizers as described above. Thus, the present invention is designed such that no quenching cracks occur while satisfying the strength properties as described above. The effect of the present invention is extremely significant in manufacturing of the solid stabilizers.

DESCRIPTION OF THE REFERENCE NUMERALS

1: solid stabilizer, 10: torsion portion, 11: arm portion, 12: ring, 13: bush, 10: eye portion 

1. A solid stabilizer fabricated by cold forming, quenching, and tempering a steel material for the solid stabilizer, the solid stabilizer characterized in that the steel material for the solid stabilizer contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B, and satisfies a condition of formula (1) below, remainder of the steel material for the solid stabilizer is formed of Fe and an unavoidable impurity, and hardness in a radial center portion of the solid stabilizer after the tempering is 400 HV or more, and a martensite ratio in the radial center portion of the solid stabilizer after the tempering is 80% or more. 1.24<(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B²)×240}<1.7   (1)
 2. The solid stabilizer according to claim 1, wherein a lower limit of the C is 0.25%, an upper limit of the Mn is 1.00%, and a lower limit of the formula (1) is 1.4.
 3. The solid stabilizer according to claim 1, wherein a condition of formula (2) below is satisfied. (Si/C)<1.5   (2)
 4. The solid stabilizer according to claim 1, wherein after the tempering, tensile strength is 1,200 MPa or more, 0.2% proof stress is 1,100 MPa or more, and an impact value at room temperature is 70 J/cm² or more.
 5. A steel material for a solid stabilizer, containing, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of B, and satisfying a condition of formula (1) below, wherein remainder of the steel material for the solid stabilizer is formed of Fe and an unavoidable impurity, and in finish rolling, the steel material for the solid stabilizer is rolled at a heating temperature of 1,000° C. or less, and hardness of the steel material for the solid stabilizer after the rolling is 200 HV or less. 1.24<(2C+0.15Si+0.4Mn+0.4Cr)×{1+(1.5B−300B²)×240}<1.7   (1)
 6. The steel material for the solid stabilizer according to claim 5, wherein a lower limit of the C is 0.25%, an upper limit of the Mn is 1.00%, and a lower limit of the formula (1) is 1.4.
 7. The steel material for the solid stabilizer according to claim 5, wherein a condition of formula (2) below is satisfied. (Si/C)<1.5   (2)
 8. A method for manufacturing a solid stabilizer, comprising: a forming step of cold bending a steel material for the solid stabilizer that contains, in mass %, 0.24 to 0.40% of C, 0.15 to 0.40% of Si, 0.50 to 1.20% of Mn, 0.03% or less of P, 0.30% or less of Cr, 0.01 to 0.03% of Ti, and 0.0010 to 0.0030% of 13, and that satisfies a condition of formula (1) below, wherein remainder of the steel material for the solid stabilizer is formed of Fe and an unavoidable impurity, and in finish rolling, the steel material for the solid stabilizer is rolled at a heating temperature of 1,000° C. or less, and hardness of the steel material for the solid stabilizer after the rolling is 200 HV or less, the method further comprising: a quenching step of quenching the steel material for the solid stabilizer after the forming; and a tempering step of tempering the steel material for the solid stabilizer after the quenching. 1.24<(2C+0.1Si+0.4Mn+0.4Cr)×{1+(1.5B−300B²)×240}<1.7   (1)
 9. The method for manufacturing the solid stabilizer according to claim 8, wherein a lower limit of the C is 0.25%, an upper limit of the Mn is 1.00%, and a lower limit of the formula (1) is 1.4.
 10. The method for manufacturing the solid stabilizer according to claim 8, wherein a condition of formula (2) below is satisfied. (Si/C)<1.5   (2)
 11. The solid stabilizer according to claim 2, wherein a condition of formula (2) below is satisfied. (Si/C)<1.5   (2)
 12. The solid stabilizer according to claim 2, wherein after the tempering, tensile strength is 1,200 MPa or more, 0.2% proof stress is 1,100 MPa or more, and an impact value at room temperature is 70 J/cm² or more.
 13. The solid stabilizer according to claim 3, wherein after the tempering, tensile strength is 1,200 MPa or more, 0.2% proof stress is 1,100 MPa or more, and an impact value at room temperature is 70 J/cm² or more.
 14. The solid stabilizer according to claim 11, wherein after the tempering, tensile strength is 1,200 MPa or more, 0.2% proof stress is 1,100 MPa or more, and an impact value at room temperature is 70 J/cm² or more.
 15. The steel material for the solid stabilizer according to claim 6, wherein a condition of formula (2) below is satisfied. (Si/C)<1.5   (2)
 16. The method for manufacturing the solid stabilizer according to claim 9, wherein a condition of formula (2) below is satisfied. (Si/C)<1.5   (2) 